Field of the Invention
The present invention concern a magnetic resonance system and method to correct phase errors in multidimensional, spatially selective radio-frequency excitation pulses, used in a magnetic resonance data acquisition sequence that operates a magnetic resonance data acquisition unit.
Description of the Prior Art
Magnetic resonance (MR) tomography is an imaging method that enables the acquisition of two-dimensional or three-dimensional image data sets that can depict structures inside an examination subject with high resolution. In MR, the magnetic moments of protons in an examination subject are aligned in a basic magnetic field or primary magnetic field (B0) so that a macroscopic magnetization appears along a longitudinal direction. This alignment is subsequently deflected out of the steady state, parallel to the basic magnetic field, by the radiation of radio-frequency (RF) pulses (excitation, TX). A transverse magnetization is thereby generated. Special RF transmission coils of an MR system are typically used for the RF radiation.
The decay of the transverse magnetization back into the steady state (the magnetization dynamic) is subsequently detected as MR data by one or more RF reception coils of the MR system (imaging, RX). A spatial coding of the acquired MR data is achieved by the application of different magnetic field gradients (for slice selection, phase coding or frequency coding). A targeted dephasing/rephasing of the transverse magnetization to achieve what is known as a gradient axis can occur by the application of gradient fields. The gradient fields can be applied along axes (gradient axes) of an apparatus coordinate system of the MR system via coils provided for this purpose. The different gradient axes can be controlled via separate channels. It is also possible to achieve a rephasing of the transverse magnetization (known as the spin echo) by the radiation of an RF pulse. The detected (and therefore spatially resolved) MR data initially exist in a mathematical arrangement in the frequency domain, known as k-space, and can be transformed into the spatial domain, known as image space by subsequent Fourier transformation. K-space can be scanned (filled with data) along different trajectories by the targeted switching (activation) of the magnetic field gradients. A conventional and widely used scan includes the successive detection of frequency-coded k-space lines for different phase codings. A corresponding coordinate system aligned to the spatial coding is designated as a phase-gradient-slice (PGS) coordinate system. In particular, the PGS coordinate system can be aligned on a patient coordinate system that determines the anatomical planes (for example transversal, sagittal and coronal planes) of the examination subject.
Recently, RF excitation pulses have been developed in an attempt to shorten the measurement time, for instance for multidimensional, selective excitation. Such RF excitation pulses use special k-space trajectories to excite the transverse magnetization. Regions that are clearly spatially defined (and bounded, for example) in two dimensions (2D) or three dimensions (3D) can thereby be excited. For example, gradient fields along multiple axes are used for this purpose. It is also possible to provide a special amplitude modulation of the RF excitation pulse.
This can in turn allow the number of sample points to be limited, and thereby reduce the time required to implement a complete measurement sequence. Examples of such pulses are known, for instance, from “Two-Dimensional Spatially-Selective RF Excitation Pulses in Echo-Planar Imaging” by S. Riesenberg et al. in Mag. Reson. Med. 47 (2002), 1186-1193. Such RF excitation pulses are known as echoplanar, spatially-selective RF excitation pulses, or can use spiral-shaped trajectories.
However, due to the greater complexity in comparison to conventional one-dimensional (1D) RF excitation pulses—for example with a constant gradient field, thus slice-selective excitation—a greater tendency toward artifact formation given system inaccuracies can occur in multidimensional, spatially-selective RF excitation pulses. In particular, phase errors can occur, i.e. incorrect phases during the RF excitation pulse.
One class of system inaccuracies relates to systematic error sources that are inherent to the system and typically exist systematically, and have no or only a slight time dependency, such as time synchronization errors of the MR systems produce artifacts and errors in the execution of the k-space trajectories during the excitation. These errors steam from systematic time shifts between the amplitude or phase of the RF excitation pulse and the gradient fields and/or between the amplitude or phase of the RF excitation pulse and the radio-frequency, for instance of a numerically controlled oscillator. Additional sources of artifacts can be gradient delay between the different gradient axes, incorrect amplitudes of the gradients, a channel-specific and/or global delay of the radio-frequency of the RF excitation pulses and the gradient fields. Such artifacts are known as “TX ghosting” or “phase mismatch”; see in this regard “Calibration of Echo-Planar 2D-Selective RF Excitation Pulses” by M. Oelhafen et al. in Mag. Reson. Med. 52 (2004), 1136-1145, and “Robust Spatially Selective Excitation Using Radiofrequency Pulses Adapted to the Effective Spatially Encoding Magnetic Fields” by J. T. Schneider et al. in Mag. Reson. Med 65 (2011), 409-421.
An additional class of system inaccuracies concerns time-variable error sources, such as component heating and system parameter drift in general, and component instabilities. The examination subject can be a source of time-dependent error sources, for example due to an overall (gross) movement of the examination subject, organ movement, or physiological variations such as heart beat, breathing or brain movement. For example, an organ movement can produce a shift of the regions of different susceptibilities, which in turn results in time-dependent, B0-dependent  artifacts. Moreover, eddy currents that develop during the application of the RF excitation pulse can affect the phases and k-space trajectory, which can produce artifacts.
The aforementioned time-dependent error sources have a particular importance to MR measurement sequences with repeated imaging, thus for instance “multislice EPI”. A transverse magnetization is repeatedly excited multiple times, for example in order to image different slices or regions. However, such MR measurement sequences can also include different repeated preparation of the transverse magnetization, such as diffusion coding, spin labeling or the use of contrast agents for functional MR (fMR), perfusion or diffusion imaging. Such measurement sequences extend over a time period of minutes.
In this regard, in many cases it is not possible (or is possible only to a limited extent) to ensure a high stability of the system parameters or the parameters of the measurement system, such that even a comprehensive, individual calibration to correct phase errors at the beginning of the measurement sequence (as is known from the aforementioned publications by J. T. Schneider et al. and M. Oelhafen et al.) can have only a limited period of validity. Moreover, these known calibration techniques have additional disadvantages: a relatively long time period is often required for the implementation of the calibration, such that the time period of the entire measurement sequence is undesirably increased. It can also be necessary to implement these calibration techniques as separate sequences (for example before the actual measurement sequence), which can make a particularly complicated implementation necessary with regard to the operation of the MR system.
Many techniques were described above with regard to the excitation of the transverse magnetization. It should also be noted that similar problems and system inaccuracies can occur with regard to the imaging: complicated k-space trajectories can be used not only during the excitation but also for the imaging. There corresponding problems and artifacts can occur, which determine a time synchronization of the amplitude and phase of the RF excitation pulse and the gradient fields, as well as the amplitude and phase of the RF excitation pulse and the radio-frequency of the RF excitation pulse. Examples of such imaging are, for example, echoplanar imaging (EPI) and variants thereof that are known to those skilled in the art under the following terms: “blipped EPI”, “spiral EPI” or radial EPI acquisition sequences.